SKF 38393, a D-1 dopamine receptor agonist, produced dose-dependent elevations in plasma prolactin concentrations. Following the administration of SCH 23390, a D-1 antagonist, plasma prolactin concentrations tended to decrease; and low doses of SCH 23390 completely prevented the SKF 38393-induced elevations in plasma prolactin. These observations suggest that D-1 receptor stimulation can promote prolactin secretion.